Hey Lion Design, I was in a similar situation as you last year. Had my G5 with both a 10.4 and 10.5 install on it. I thought 10.4 running FreeHand MX was the best but it was long-in-the-tooth for so many other applications that 10.5 became my more used OS. To be honest, I'm still on 10.5 with a newer Mac Pro Quad-core Intel but only because I can't let go of some utilities that I depend on that are not available on Snow Leopard.
I guess what you should get depends on the applications you use. For example, I will keep Photoshop, InDesign/Quark, FreeHand, Mail, Safari, and more all running at the same time so the multi-tasking is important. You want enough processor and RAM muscle to make your work easy. I like the Mac Pro but a good buddy of mine (who also runs multiple programs for web design and photography) decided on the new 27" iMac Quad-core and raves about it. He too traded in his G5 for this. You can't install extra internal drives like you can with the Mac Pro but you can attached Firewire 800 and Thunderbolt devices externally.
There is plenty of help on this forum to keep you running with FreeHand, no matter what you get, as long as it isn't running Lion 10.7. But that is what Free FreeHand is fighting for anyway; a future no matter the OS.